1 //===- XRayInstrumentation.cpp - Adds XRay instrumentation to functions. --===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements a MachineFunctionPass that inserts the appropriate
11 // XRay instrumentation instructions. We look for XRay-specific attributes
12 // on the function to determine whether we should insert the replacement
13 // operations.
14 //
15 //===---------------------------------------------------------------------===//
16 
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Triple.h"
20 #include "llvm/CodeGen/MachineBasicBlock.h"
21 #include "llvm/CodeGen/MachineDominators.h"
22 #include "llvm/CodeGen/MachineFunction.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstrBuilder.h"
25 #include "llvm/CodeGen/MachineLoopInfo.h"
26 #include "llvm/CodeGen/TargetInstrInfo.h"
27 #include "llvm/CodeGen/TargetSubtargetInfo.h"
28 #include "llvm/IR/Attributes.h"
29 #include "llvm/IR/Function.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Target/TargetMachine.h"
32 
33 using namespace llvm;
34 
35 namespace {
36 
37 struct InstrumentationOptions {
38   // Whether to emit PATCHABLE_TAIL_CALL.
39   bool HandleTailcall;
40 
41   // Whether to emit PATCHABLE_RET/PATCHABLE_FUNCTION_EXIT for all forms of
42   // return, e.g. conditional return.
43   bool HandleAllReturns;
44 };
45 
46 struct XRayInstrumentation : public MachineFunctionPass {
47   static char ID;
48 
49   XRayInstrumentation() : MachineFunctionPass(ID) {
50     initializeXRayInstrumentationPass(*PassRegistry::getPassRegistry());
51   }
52 
53   void getAnalysisUsage(AnalysisUsage &AU) const override {
54     AU.setPreservesCFG();
55     AU.addPreserved<MachineLoopInfo>();
56     AU.addPreserved<MachineDominatorTree>();
57     MachineFunctionPass::getAnalysisUsage(AU);
58   }
59 
60   bool runOnMachineFunction(MachineFunction &MF) override;
61 
62 private:
63   // Replace the original RET instruction with the exit sled code ("patchable
64   //   ret" pseudo-instruction), so that at runtime XRay can replace the sled
65   //   with a code jumping to XRay trampoline, which calls the tracing handler
66   //   and, in the end, issues the RET instruction.
67   // This is the approach to go on CPUs which have a single RET instruction,
68   //   like x86/x86_64.
69   void replaceRetWithPatchableRet(MachineFunction &MF,
70                                   const TargetInstrInfo *TII,
71                                   InstrumentationOptions);
72 
73   // Prepend the original return instruction with the exit sled code ("patchable
74   //   function exit" pseudo-instruction), preserving the original return
75   //   instruction just after the exit sled code.
76   // This is the approach to go on CPUs which have multiple options for the
77   //   return instruction, like ARM. For such CPUs we can't just jump into the
78   //   XRay trampoline and issue a single return instruction there. We rather
79   //   have to call the trampoline and return from it to the original return
80   //   instruction of the function being instrumented.
81   void prependRetWithPatchableExit(MachineFunction &MF,
82                                    const TargetInstrInfo *TII,
83                                    InstrumentationOptions);
84 };
85 
86 } // end anonymous namespace
87 
88 void XRayInstrumentation::replaceRetWithPatchableRet(
89     MachineFunction &MF, const TargetInstrInfo *TII,
90     InstrumentationOptions op) {
91   // We look for *all* terminators and returns, then replace those with
92   // PATCHABLE_RET instructions.
93   SmallVector<MachineInstr *, 4> Terminators;
94   for (auto &MBB : MF) {
95     for (auto &T : MBB.terminators()) {
96       unsigned Opc = 0;
97       if (T.isReturn() &&
98           (op.HandleAllReturns || T.getOpcode() == TII->getReturnOpcode())) {
99         // Replace return instructions with:
100         //   PATCHABLE_RET <Opcode>, <Operand>...
101         Opc = TargetOpcode::PATCHABLE_RET;
102       }
103       if (TII->isTailCall(T) && op.HandleTailcall) {
104         // Treat the tail call as a return instruction, which has a
105         // different-looking sled than the normal return case.
106         Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
107       }
108       if (Opc != 0) {
109         auto MIB = BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc))
110                        .addImm(T.getOpcode());
111         for (auto &MO : T.operands())
112           MIB.add(MO);
113         Terminators.push_back(&T);
114       }
115     }
116   }
117 
118   for (auto &I : Terminators)
119     I->eraseFromParent();
120 }
121 
122 void XRayInstrumentation::prependRetWithPatchableExit(
123     MachineFunction &MF, const TargetInstrInfo *TII,
124     InstrumentationOptions op) {
125   for (auto &MBB : MF)
126     for (auto &T : MBB.terminators()) {
127       unsigned Opc = 0;
128       if (T.isReturn() &&
129           (op.HandleAllReturns || T.getOpcode() == TII->getReturnOpcode())) {
130         Opc = TargetOpcode::PATCHABLE_FUNCTION_EXIT;
131       }
132       if (TII->isTailCall(T) && op.HandleTailcall) {
133         Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
134       }
135       if (Opc != 0) {
136         // Prepend the return instruction with PATCHABLE_FUNCTION_EXIT or
137         //   PATCHABLE_TAIL_CALL .
138         BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc));
139       }
140     }
141 }
142 
143 bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
144   auto &F = MF.getFunction();
145   auto InstrAttr = F.getFnAttribute("function-instrument");
146   bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
147                           InstrAttr.isStringAttribute() &&
148                           InstrAttr.getValueAsString() == "xray-always";
149   Attribute Attr = F.getFnAttribute("xray-instruction-threshold");
150   unsigned XRayThreshold = 0;
151   if (!AlwaysInstrument) {
152     if (Attr.hasAttribute(Attribute::None) || !Attr.isStringAttribute())
153       return false; // XRay threshold attribute not found.
154     if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
155       return false; // Invalid value for threshold.
156 
157     // Count the number of MachineInstr`s in MachineFunction
158     int64_t MICount = 0;
159     for (const auto &MBB : MF)
160       MICount += MBB.size();
161 
162     // Get MachineDominatorTree or compute it on the fly if it's unavailable
163     auto *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
164     MachineDominatorTree ComputedMDT;
165     if (!MDT) {
166       ComputedMDT.getBase().recalculate(MF);
167       MDT = &ComputedMDT;
168     }
169 
170     // Get MachineLoopInfo or compute it on the fly if it's unavailable
171     auto *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
172     MachineLoopInfo ComputedMLI;
173     if (!MLI) {
174       ComputedMLI.getBase().analyze(MDT->getBase());
175       MLI = &ComputedMLI;
176     }
177 
178     // Check if we have a loop.
179     // FIXME: Maybe make this smarter, and see whether the loops are dependent
180     // on inputs or side-effects?
181     if (MLI->empty() && MICount < XRayThreshold)
182       return false; // Function is too small and has no loops.
183   }
184 
185   // We look for the first non-empty MachineBasicBlock, so that we can insert
186   // the function instrumentation in the appropriate place.
187   auto MBI = llvm::find_if(
188       MF, [&](const MachineBasicBlock &MBB) { return !MBB.empty(); });
189   if (MBI == MF.end())
190     return false; // The function is empty.
191 
192   auto *TII = MF.getSubtarget().getInstrInfo();
193   auto &FirstMBB = *MBI;
194   auto &FirstMI = *FirstMBB.begin();
195 
196   if (!MF.getSubtarget().isXRaySupported()) {
197     FirstMI.emitError("An attempt to perform XRay instrumentation for an"
198                       " unsupported target.");
199     return false;
200   }
201 
202   // First, insert an PATCHABLE_FUNCTION_ENTER as the first instruction of the
203   // MachineFunction.
204   BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
205           TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
206 
207   switch (MF.getTarget().getTargetTriple().getArch()) {
208   case Triple::ArchType::arm:
209   case Triple::ArchType::thumb:
210   case Triple::ArchType::aarch64:
211   case Triple::ArchType::mips:
212   case Triple::ArchType::mipsel:
213   case Triple::ArchType::mips64:
214   case Triple::ArchType::mips64el: {
215     // For the architectures which don't have a single return instruction
216     InstrumentationOptions op;
217     op.HandleTailcall = false;
218     op.HandleAllReturns = true;
219     prependRetWithPatchableExit(MF, TII, op);
220     break;
221   }
222   case Triple::ArchType::ppc64le: {
223     // PPC has conditional returns. Turn them into branch and plain returns.
224     InstrumentationOptions op;
225     op.HandleTailcall = false;
226     op.HandleAllReturns = true;
227     replaceRetWithPatchableRet(MF, TII, op);
228     break;
229   }
230   default: {
231     // For the architectures that have a single return instruction (such as
232     //   RETQ on x86_64).
233     InstrumentationOptions op;
234     op.HandleTailcall = true;
235     op.HandleAllReturns = false;
236     replaceRetWithPatchableRet(MF, TII, op);
237     break;
238   }
239   }
240   return true;
241 }
242 
243 char XRayInstrumentation::ID = 0;
244 char &llvm::XRayInstrumentationID = XRayInstrumentation::ID;
245 INITIALIZE_PASS_BEGIN(XRayInstrumentation, "xray-instrumentation",
246                       "Insert XRay ops", false, false)
247 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
248 INITIALIZE_PASS_END(XRayInstrumentation, "xray-instrumentation",
249                     "Insert XRay ops", false, false)
250